Many thanks Chris for the explanation and solution.
That indeed did solve the problem. In my actual case I was writing Text3D
characters on the inside and outside of the cylinder and then rotating it
and I wanted the characters to be quite close to the surface.
--
David Park
djmpark at comcast.net
http://home.comcast.net/~djmpark/
"Chris Hill" <chill at wolfram.com> wrote in message
news:fbqr1t$7n7$1 at smc.vnet.net...
> At 04:38 AM 9/6/2007, David Park wrote:
>>The following illustrates a strange behavior of surfaces with respect to
>>opacity. The code draws a narrow cylinder with a red line on the inside of
>>the cylinder and a black line on the outside of the cylinder. The cylinder
>>is completely opaque. The inside red line does not show through the
>>cylinder, which is proper behavior. However, the black outside line does
>>show through the back face for a considerable range, which is certainly
>>improper behavior.
>>
>>cylinder =
>> First[ParametricPlot3D[{5 Cos[\[Theta]1], 5 Sin[\[Theta]1],
>> z}, {\[Theta]1, 0, 2 \[Pi]}, {z, -1, 1},
>> PlotStyle -> {FaceForm[Orange, LightGray]},
>> PlotPoints -> {41, 5},
>> Mesh -> None]];
>>
>>outsideline =
>> First[ParametricPlot3D[{5.1 Cos[\[Theta]1], 5.1 Sin[\[Theta]1],
>> 0.2}, {\[Theta]1, 0, 2 \[Pi]},
>> PlotStyle -> {Black},
>> PlotPoints -> 41]];
>>
>>insideline =
>> First[ParametricPlot3D[{4.9 Cos[\[Theta]1],
>> 4.9 Sin[\[Theta]1], -0.2}, {\[Theta]1, 0, 2 \[Pi]},
>> PlotStyle -> {Red},
>> PlotPoints -> 41]];
>>
>>Show[Graphics3D[
>> {cylinder,
>> outsideline,
>> insideline}],
>> Lighting -> "Neutral",
>> ViewPoint -> {1.718, -2.747, 0.975},
>> Boxed -> False
>
> As part of the default hidden surface removal algorithm, Mathematica
> pushes lines towards the viewer to achieve the correct
> sorting. Given a polygon and a line which are coplanar and
> overlapping this makes the line draw consistently in front of the
> polygon as is expected. In some cases such the one you've
> discovered, this can also incorrectly make lines that are behind
> polygons appear to be in front.
>
> The problem is not related to the sides of the surface. The black
> line happens to poke though the cylinder when it passes behind the
> cylinder at the part of the cylinder that is most distant from the
> view point (the red line passes behind the cylinder closer to the
> view point). Inaccuracies of this nature are more likely to be
> observed at a greater distance from the view point all other things
> being equal.
>
> One way to avoid the problem you have encountered is to introduce
> transparency which causes Mathematica to automatically use a
> different hidden surface removal algorithm that is slower and
> consumes more memory but that better handles cases like this. Here I
> use Opacity[0.999] which is enough transparency to trigger a change
> in the hidden surface algorithm, but not so much as to make objects
> in the scene appear transparent.
>
> Show[Graphics3D[{Opacity[0.999], cylinder, outsideline, insideline}],
> Lighting -> "Neutral", ViewPoint -> {1.718, -2.747, 0.975}, Boxed ->
> False]
>
> Chris Hill
> Wolfram Research
>
>